Electrical transmission and distribution networks consist of a staggering number of transformers, capacitor banks, reactors, motors, generators and other major pieces of electrical equipment. Such equipment is extremely expensive. Further, each such piece of equipment typically plays a vital role in the distribution of power to end users, such that an outage caused by the equipment being damaged or taken out of service for repairs, may have exceptionally costly consequences. As a result, such equipment is typically protected from potentially damaging overvoltages and overcurrents by protective components, such as fuses and surge attesters.
A fuse is a current interrupting device which includes a current-responsive element that will melt or fuse open when an overcurrent or short circuit of a predetermined magnitude and duration is conducted through the fuse. After the fuse has operated to interrupt the overcurrent, it must be replaced in order to restore service.
A particularly convenient and desirable fusing device is a dropout fuseholder. A dropout fuseholder includes a pair of terminals for connecting the fuseholder to the circuit that is to be protected, and an actuation means for causing the fuseholder to physically drop out of engagement with the energized circuit after the fuse has operated to clear a fault. The drop open feature provides a clear visual indication to utility personnel that the fuse has operated. Further, the drop open feature removes the fuseholder from the voltage stress otherwise associated with the energized circuit, eliminating the possibility of tracking and ultimate flashover around the fuse.
A typical prior art dropout fuseholder is disclosed in U.S. Pat. No. 3,611,240 (Mikulecky). Mikulecky discloses a current-limiting dropout fuseholder having a full range of fault clearing capability. As disclosed in that patent, upon actuation of the fuse, an explosive charge is ignited which actuates the dropout mechanism and frees the fuseholder to drop out of engagement with the cutout mounting in which it is installed. Similarly, U.S. Pat. No. 3,825,871 (Blewitt) also discloses a dropout fuseholder which employs an explosive charge to initiate dropout.
Although such explosive charges have generally been successfully employed in the industry, it is not uncommon for such a fuse to fail to drop open after clearing a fault due to a failure of the charge to detonate. Such failure is frequently due to the powder of the explosive charge absorbing too much moisture to ignite after a prolonged period of service. No matter the reason for such failures, the failure of a dropout fuse to drop open after operation is a source of great frustration and delay to utility personnel who are unable to identify the actuated fuses by visual observation, and must instead resort to more time consuming and less convenient means for detecting which fuses have operated. Further, a fuseholder that has failed to drop open remains subject to the voltage stress imposed by the energized network, making it susceptible to tracking and possible flashover.
Accordingly, despite advances made in fuse technology over the years, further improvements would be welcomed by the industry. Specifically, there is a need for a mechanically operable dropout mechanism that would retain the fuseholder in its current-carrying position until actuation of the fusible element, and would reliably cause the fuse to drop open upon actuation of the fuse. The dropout mechanism should allow the fuse to completely clear the fault before dropout is initiated. Ideally, the device would be simple and inexpensive to manufacture, and would provide dependable operation without the need to rely upon an explosive charge.